A turboprop engine comprises an air input, compressors, a combustion chamber, a turbine, a propeller and a speed reducer. Placed between the turbine and the axis of the propeller, a speed reducer is used to reduce the speed of rotation of the propeller compared to the speed of rotation of the shaft of the turbine, said speed of rotation of the turbine being too fast to supply said propeller.
Numerous types of speed reducers exist, notably epicyclic gear train reducers, chain reducers, worm screw reducers, intermediate drive line reducers, etc.
In a turboprop engine, the use of a reducer with two intermediate lines is particularly interesting because such a reducer allows an important reduction in speed, in a confined space and with a controlled mass. A reducer with two intermediate lines 1 is schematically represented in FIG. 1. With reference to FIG. 1, the reducer 1 comprises:                an input line 16 comprising an input gear wheel 10 driven by an input shaft a1,        a first intermediate line 17 comprising a first intermediate gear wheel 11 and a third intermediate gear wheel 13 driven by a first intermediate shaft a2,        a second intermediate line 18 comprising a second intermediate gear wheel 11 and a fourth intermediate gear wheel 14 driven by a second intermediate shaft a3,        an output line 19 comprising an output gear wheel 15 driven by an output shaft a4.        
When the reducer 1 is used in a turboprop engine, the output shaft a4 of the output line 19 is the shaft of the propeller and the input shaft a1 of the input line is the shaft of the turbine. Thus, the speed of rotation of the output shaft a4 is reduced compared to the speed of rotation of the input shaft a1. To do so, half of the power of the input shaft has to transit through each of the intermediate shafts a2 and a3 to finally be transmitted to the output shaft a4. The transmission of power takes place by means of gear wheels meshed with each other.
However, such reducers are hyperstatic systems, that is to say that without particular arrangement it is possible that an intermediate line transmits the majority of the engine power, whereas the other intermediate line practically does not transmit any power. Indeed, it is difficult to guarantee optimal meshing of the gear wheels with each other and thereby equal distribution of the power of the input gear wheel 10 between the first intermediate gear wheel 11 and the second intermediate gear wheel 12 and, consequently, between the third intermediate gear wheel 13 and the fourth intermediate gear wheel 14. FIG. 2 represents a front view of the speed reducer with two intermediate lines 1 presented in FIG. 1. With reference to FIG. 2, even if the input gear wheel 10 is in contact with the first intermediate gear wheel 11 and the second intermediate gear wheel 12 at the level, respectively, of the points A and B, it is difficult to guarantee that there is no play present at the level of the points C and D. It should be noted that the points C and D correspond to the contact points, respectively, of the third intermediate gear wheel 13 with the output gear wheel 15 and the fourth intermediate gear wheel 14 with the output gear wheel 15.
Thus, a so-called “load distribution” or “torque distribution” system is necessary to ensure that half of the power transits via each of the intermediate lines 17 and 18.
For example, U.S. Pat. No. 1,351,321A describes a load distribution system by the addition of a degree of vertical freedom to an input gear wheel by means of a rectangular part that can slide vertically. Said degree of freedom allows said input gear wheel to position itself freely along a vertical axis and thereby to ensure good distribution of the torque between the intermediate transmission lines. The compression spring, positioned below the rectangular part, has the function of compensating the vertical force due to the weight of the gear wheel.
However, the problem of such a “load distribution” system is that the use of the rectangular part mounted on the compression spring is complex to produce and risks degrading the reliability of the speed reducer. Moreover, the system according to the prior art does not allow the input gear wheel to move in an optimal manner.